Campaign finance

Democratic presidential candidate former South Bend Mayor Pete Buttigieg participates in a Democratic presidential primary debate at the Gaillard Center, Tuesday, Feb. 25, 2020, in Charleston, S.C., co-hosted by CBS News and the Congressional Black Caucus Institute. (AP Photo/Patrick Semansky)

Democratic presidential candidate Sen. Bernie Sanders, I-Vt., speaks during a campaign event in San Antonio, Saturday, Feb. 22, 2020. (AP Photo/Eric Gay)

FILE - In this June 13, 2019, file photo, Steve Marcinkus, an Investigator with the Office of the City Commissioners, demonstrates the ExpressVote XL voting machine at the Reading Terminal Market in Philadelphia. Nearly 1 in 5 U.S. voters will cast ballots this year on devices that look and feel like the discredited paperless voting machines they once used, yet leave a paper record of the vote. Computer security experts are warning that these so-called ballot-marking devices pose too much of a risk. Ballot-marking machines were initially developed not as primary vote-casting tools but as “accessible” alternatives for the disabled. They print out paper records that are scanned by optical readers that tabulate the vote. They cost at least twice as much as hand-marked paper ballots, which computer scientists prefer because paper can’t be hacked. (AP Photo/Matt Rourke, File)

FILE - In this Feb. 11, 2020, file photo, Senate Majority Leader Mitch McConnell, R-Ky., talks to reporters following a GOP strategy meeting at the Capitol in Washington. A political committee linked to Senate Majority Leader Mitch McConnell is funding ads and other campaign materials designed to meddle in North Carolina's Democratic Senate primary, the group acknowledged on Friday. (AP Photo/J. Scott Applewhite, File)

FILE - In this Feb. 19, 2020, file photo, people dressed as showgirls walk near the Paris Las Vegas hotel casino, site of a Democratic presidential debate in Las Vegas. Just past the roulette wheel and slot machines, the smoky bars and blinking lights, Nevada Democrats are preparing to weigh in on their party's presidential nomination fight. (AP Photo/John Locher, File)

Democratic presidential candidate and former South Bend Mayor Pete Buttigieg takes questions from students at the USC Dornsife Center for the Political Future town hall in Los Angeles Thursday, Feb. 20, 2020. (AP Photo/Damian Dovarganes)

Democratic presidential candidate Sen. Elizabeth Warren, D-Mass., speaks with Bill Mamgum, center, and Shirley Mamgum at EllaEm's Soul Food, Thursday, Feb. 20, 2020, in North Las Vegas, Nev. (AP Photo/John Locher)

FILE - In this Dec. 21, 2019, file photo, Democratic presidential candidate Sen. Bernie Sanders, I-Vt., speaks during a rally in Venice, Calif. California is the largest prize in the calculations of any Democratic presidential candidate, but it rarely seems that way. (AP Photo/Kelvin Kuo, File)

From left, Democratic presidential candidates, former New York City Mayor Mike Bloomberg, Sen. Elizabeth Warren, D-Mass., Sen. Bernie Sanders, I-Vt., participate in a Democratic presidential primary debate Wednesday, Feb. 19, 2020, in Las Vegas, hosted by NBC News and MSNBC. (AP Photo/John Locher)

Democratic presidential candidate and former New York City Mayor Mike Bloomberg waves after speaking at a campaign event, Thursday, Feb. 20, 2020, in Salt Lake City. (AP Photo/Rick Bowmer)